Alloy



Patented Sept. 25, 1934 UNITED STATES PATENT OFFICE ALLOY No Drawing. Application August 17, 1933,- Serial No. 685,643

3 'Claims.

This invention relates to aluminum base alloys adapted to being mechanically worked. It is particularly concerned with an improvement in wrought aluminum-magnesium alloys.

It is commonly recognized by those skilled in the art of working metals that a small uniform grain size is very desirable in stock which is to be worked. While such a condition is eminently desirable it is frequently difficult to obtain either because of the nature of the alloy or the fabricating schedule that must be followed in commercial practice. Among aluminum base alloys, the aluminum-magnesium class is particularly susceptible to the development of coarse grains when thermally treated. This class of alloys has recently been. recognized as possessing very desirable properties of strength and corrosion resistance, but it also has a tendency to develop a coarse grain structure which is distinctly disadvantageous, especially if the annealed or heat-treated material is subsequently subjected to severe working operations such as drawing into cooking utensils. My invention is concerned with a means for overcoming this undesirable feature of aluminummagnesium alloys.

It is an object of my invention to produce a small and uniform grain size in aluminum-magnesium alloys without deleteriously affecting other properties. A further object is to obtain a fine-grained structure in thermally treated aluminum-magnesium alloys under the usual conditions of commercial operation. These objects are accomplished by the addition to the alloy of a substance which is readily available and which can be introduced without special precautions.

The aluminum-magnesium alloys employed in making wrought articles usually contain from 2 to 15 per cent of magnesium. It is now recognized that the use of high purity aluminum containing less than 0.3 per cent impurities serves to enhance the tensile properties and corrosion resistance of the alloy. In using high purity metal, however, there is an increased tendency to form a coarsegrained structure when the alloy is heated after having been worked. I have found that the addition of from about 0.05 to 0.2 per cent molybdenum to aluminum-magnesium alloys, especially those alloys made of high purity metal, produces a uniformly fine-grained structure in the annealed or heat-treated material. The improvement wrought by the addition of molybdenum is evident in the smooth and pleasing surface obtained in drawn articles such as cooking utensils. The molybdenum addition also serves to increase the tensile and yield strengths without diminishing the ductility to an appreciable extent. It has furthermore been observed that alloys containing molybdenum are somewhat more resistant to corrosion than the alloys without this element.

In alloys containing between 2 and 8 per cent magnesium I have found that the presence of 0.05 to 0.15 per cent molybdenum is sufilcient to impart the desired properties to the alloy. When more than 8 per cent magnesium is used, it is necessary to use from about 0.10 to 0.20 per cent of the added element to secure a fine-grained structure. If more than about 0.2 per cent of molybdenum is employed, an undesirable laminated structure is developed in the alloy when it is worked.

The alloys containing from 2 to 8 per cent magnesium are better adapted to severe working than alloys having larger amounts of magnesium and hence the beneficial effect of molybdenum is most noticeable within this range of composi tion. I have found that an alloy containing 6 per cent magnesium and made from aluminum having a total of 0.3 per cent the impurities iron and silicon, when rolled to a sheet thickness of .064 inch, annealed and drawn into pans, develops a hard, rough surface which is exceedingly diflicult to polish by the ordinary methods of bufling. The same material with the additionof 0.1 per cent molybdenum when annealed and drawn into a pan had a smooth, easily polishable surface. It was further determined that the pans containing no molybdenum showed more evidence of corrosive attack when exposed to tap water over an extended period of time than the pans made of the alloy containing molybdenum.

When used for structural purposes, aluminummagnesium alloys are usually exposed to elevated temperatures or heat-treated to develop the maximum strength. Such treatment is a severe test of the grain growth tendencies in an alloy.

As an example of the influence of molybdenum on the physical properties of the heat-treated alloy, the following test results are cited. An alloy containing 6 per cent magnesium and a total of 0.2 per cent iron and silicon was heattreated at 800 Fahrenheit and quenched in cold water. The alloy when tested had a tensile strength of 39,860 pounds per square inch, a yield strength of 16,640 pounds per square inch and an elongation of 26.8 per cent in 2 inches. The same alloy with the addition of 0.1 per cent molybdenum and the same heat-treatment had a tensile strength of 42,430 pounds per square inch, a yield strength of 19,390 pounds per square inch and an elongation of 29.0 per cent. The

latter alloy also had a uniformly fine-grained structure.

The beneficial efiect of molybdenum is not confined to improvement in tensile properties but is also evidenced by an increased corrosion resistance as the following test data indicates. The test employed consisted of subjecting tensile test specimens to a severe corroding medium,' an aqueous solution of 3 per cent sodium chloride and 0.5 per cent hydrogen peroxide, by alternately immersing and removing the specimens from the solution. Specimens of the alloys containing 6 per cent magnesium, and 6 per cent magnesium plus 0.1 per cent molybdenum having the tensile properties in the uncorroded condition mentioned hereinabove, were found to have the following properties after being corroded 48 hours by alternate immersion in the sodium chloride-hydrogen peroxide solution. The alloy without molybdenum had a tensile strength of 34,170 pounds per square inch, a yield strength of 16,580 pounds per square inch, and an elongation of 12.1 per cent. On the other hand, the alloy containing molybdenum had a tensile strength of 41,730 pounds per square inch, a yield strength of 19,750 pounds per square inch and an elongation of 22.8 per cent. The effect of corrosion is most noticeable in the greater loss in tensile strength and elongation of the alloy having no molybdenum.

The alloys containing molybdenum may be made according to the usual alloy production methods. The molybdenum may be added to the aluminum-magnesium alloy in any known manner, but I preferably employ a rich aluminum-molybdenum alloy for this purpose. The element in any form may be added to the metal bath at a time suflicient to allow complete diffusion during the melting operations without danger of oxidation or segregation.

I claim:

1. An aluminum-magnesium alloy containing from about 2 to about 15 per cent by weight of magnesium and from about 0.05 to about 0.2 per cent by weight of molybdenum and characterized by a small and uniform grain size.

2. An aluminum-magnesium alloy containing from about 2 to about 8 per cent by weight of magnesium and from about 0.05 to about 0.15 per cent by weight of molybdenum and characterized in the wrought condition by a uniform grain which is small in size.

3. An aluminum-magnesium alloy containing from about 8 to about 15 per cent by weight of magnesium and from about 0.1 to about 0.2 per cent by weight of molybdenum and capable of retaining its small and uniform grain size upon exposure to elevated temperatures.

JOSEPH A. NOCK, JR. 

